The code is tested and working fine.
Place this code
<?php
$posttags = get_the_tags();
if( ! empty( $posttags ) ) :
$tags = array();
foreach($posttags as $key => $value){
$tags[] = $value->name;
}
?>
<meta name="keywords" content="<?php echo implode( ',', $tags ); ?>,test"/>
<?php endif;?>
instead of
<meta name="keywords" content="<?php the_tags( '', ',', '' );?>test<?php }?>"/>